Hydrogen absorbing alloy powder and electrodes formed of the hydrogen absorbing alloy powder

ABSTRACT

Provided is a hydrogen absorbing alloy powder for use in nickel-metal hydride storage batteries having a high capacity, excellent initial characteristics and a long life. Specifically provided is a process for the production of a hydrogen absorbing alloy powder which includes the steps of treating a hydrogen absorbing alloy powder with an acid solution, and subsequently treating the hydrogen absorbing alloy powder with a solution containing a condensed phosphoric acid having 2 to 20 phosphorus atoms per molecule and/or phytic acid, as well as an electrode formed of the hydrogen absorbing alloy powder produced by the above process. Also provided is a hydrogen absorbing alloy powder of the AB 5  type in which A is exothermic metal and B is endothermic metal, the hydrogen absorbing alloy powder being obtained by providing a hydrogen absorbing alloy in which A comprises a mixture composed of 20 to 50% by weight of Ce, 20 to 50% by weight of La, and the balance comprising rare earth elements other than Ce and La and inclusive of Y (yttrium), treating the hydrogen absorbing alloy powder with an acid solution, and further treating the hydrogen absorbing alloy powder with a solution containing a phosphorus compound, as well as an electrode formed of this hydrogen absorbing alloy powder.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a hydrogen absorbing alloy powder andelectrodes formed of the same. More particularly, it relates to ahydrogen absorbing alloy powder which can suitably be used to form thenegative electrodes of nickel-metal hydride storage batteries, and toelectrodes formed of the same. Still more particularly, it relates to ahydrogen absorbing alloy powder for use in nickel-metal hydride storagebatteries having a high capacity, excellent initial characteristics anda long life.

[0003] 2. Description of the Related Art

[0004] Since the discovery of hydrogen absorbing alloys capable ofabsorbing and releasing hydrogen, they not only have been used ashydrogen storage means, but also have been more and more widely appliedto heat pumps, batteries and the like. In particular, alkaline storagebatteries using a hydrogen absorbing alloy powder for the negativeelectrode have already been put to practical use, and various attemptshave successively been made to improve the capacity and life of thebatteries in view of the hydrogen absorbing alloy used therefor.

[0005] That is, with respect to the initially investigated LaNi₅ alloyhaving a CaCu₅ type crystal structure, an improvement in capacity andlife has been achieved by replacing a part of La with Ce, Pr, Nd andother rare earth elements to form misch metal (Mm), or by replacing apart of Ni with a metallic element such as Al, Co or Mn.

[0006] However, when such hydrogen absorbing alloys are used to formelectrodes for batteries, the capacity and life of the resultingbatteries can be improved, but their initial characteristics arereduced.

[0007] Generally, the capacity tends to become higher as the La contentin Mm is increased. When the La content is 100%, a maximum capacity isobtained, but the cycle life becomes extremely poor. Moreover, when apart of Ni is replaced with Co, Mn, Al or the like in order to achieve along life and prevent passivation, the equilibrium pressure is loweredto cause a reduction in low-temperature characteristics and high ratedischarge characteristics. Accordingly, in order to bring about animprovement in life, it has conventionally been proposed to use Mm inwhich a part of La is replaced with the other light rare earth element(Ce, Pr or Nd), decrease the Ce content of Mm (Japanese PatentProvisional Publication No. 62-223971/'87), or, on the contrary, add Cethereto positively (Japanese Patent Provisional Publication No.6-96766/'94). However, it has been impossible to obtain a battery havinga high capacity, a long life and excellent characteristics solely byresorting to an improvement in alloy composition.

[0008] Initial characteristics are generally expressed in terms of thenumber of charging-discharging cycles repeated until a maximum capacityis reached, and they are considered to be higher as the number of cyclesis smaller.

[0009] Usually, initial characteristics are evaluated by the capacity inthe first cycle. However, a battery having low initial characteristicshas the disadvantage that, when it is fabricated into a sealed type one,the balance between the positive and negative electrodes is lost tocause a reduction in the capacity and life of the battery.

[0010] In order to overcome the above-described disadvantages, it hasbeen conventional practice to treat a hydrogen absorbing alloy with anaqueous alkaline solution or inorganic acid. However, the alkalitreatment has been disadvantageous in that treating conditions such as ahigh concentration and a high temperature are required and it isdifficult to wash the treated hydrogen absorbing alloy with water. Inaddition, the hydrogen absorbing alloy treated by the alkaline solutionor by inorganic acid undesirably undergoes a compositional changebecause the alloy is subject not only to surface oxidation duringtreatment but also to oxidation during drying and storage. Hence, thedifficulty in handleability and safety makes a procedure for formingnegative electrodes for batteries more complicated.

SUMMARY OF THE INVENTION

[0011] Accordingly, an object of the present invention is to provide anegative electrode for batteries which is formed of a hydrogen absorbingalloy, exhibits excellent initial characteristics, and is suitable forthe fabrication of a sealed type battery having a high capacity and along life, as well as a hydrogen absorbing alloy used therefor.

[0012] In order to overcome the above-described disadvantages, thepresent inventors made intensive investigations on hydrogen absorbingalloy powders for use in negative electrodes and processes for producingthem. As a result, it was found that a hydrogen absorbing alloy powderhaving excellent storability and handleability can be easily produced bytreating a hydrogen absorbing alloy powder with a solution containing acondensed phosphoric acid and/or phytic acid, and that, when thishydrogen absorbing alloy powder is used to form the negative electrodesof nickel-metal hydride storage batteries, the initial characteristicsof the resulting batteries can be improved without reducing theircapacity or life. This technique has already been proposed (JapanesePatent Provisional Publication No. 10-176201/'98). However, there stillremains a need for further improvement in initial capacity and dischargecapacity.

[0013] Accordingly, an object of the present invention is to provide aprocess for the production of a hydrogen absorbing alloy powder havingexcellent storability and handleability, as well as a negative electrodefor use in nickel-metal hydride storage batteries which has excellentinitial characteristics and a long life.

[0014] The present inventors made further investigations on theabove-described treatment with a condensed phosphoric acid, and have nowfound that a further improvement in initial characteristics can beachieved by treating a hydrogen absorbing alloy powder with an acidsolution prior to the treatment with a condensed phosphoric acid. Thepresent invention has been completed on the basis of this finding.

[0015] Thus, the present invention relates to a process for theproduction of a hydrogen absorbing alloy powder which includes the stepsof treating a hydrogen absorbing alloy powder with an acid solution, andsubsequently treating the hydrogen absorbing alloy powder with asolution containing a phosphoric compound, especially a condensedphosphoric acid having 2 to 20 phosphorus atoms per molecule and/orphytic acid, and to an electrode formed of the hydrogen absorbing alloypowder produced by this process.

[0016] It has been found that, when a hydrogen absorbing alloy in whichmost of the rare earth elements present therein comprise Ce and La istreated with an acid solution, washed with water, and treated with aphosphorus compound, the initial characteristics of the resultingbatteries can be improved more without reducing their capacity or life.The present invention has been completed on the basis of this finding.

[0017] More specifically, the present inventors made investigations withattention paid to the fact that the Ce content in Mm is an importantpoint for the improvement of life and other characteristics, and havenow found that, when a certain proportion of La present in a hydrogenabsorbing alloy is replaced with Ce, and the hydrogen absorbing alloypowder is treated with an acid solution and then with a phosphoruscompound to achieve a high initial capacity and a long life, negativeelectrodes for use in nickel-metal hydride storage batteries which havea high capacity and a long life can be formed by using the resultinghydrogen absorbing alloy. Thus, the present invention has beencompleted.

[0018] The above objects of the present invention are accomplished by ahydrogen absorbing alloy powder of the AB₅ type in which A is exothermicmetal and B is endothermic metal, the hydrogen absorbing alloy powderbeing obtained by providing a hydrogen absorbing alloy in which Apreferably comprises a mixture of rare earth elements containing 20 to50% by weight of Ce and 20 to 50% by weight of La, treating the hydrogenabsorbing alloy with an acid solution, and subsequently treating thehydrogen absorbing alloy with a phosphoric compound.

[0019] The hydrogen absorbing alloy powder of the present invention,when used to form the negative electrodes of nickel-metal hydridestorage batteries, exhibits excellent initial characteristics and,moreover, can yield hydrogen absorbing alloy negative electrodes whichare suitable for the fabrication of sealed type batteries having a longcycle life.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] No particular limitation is placed on the type of the hydrogenabsorbing alloy used in the present invention, and it may be suitablyselected from among various well-known hydrogen absorbing alloys used toform negative electrodes. In order to improve cycle life characteristicsespecially when it is used for batteries, hydrogen absorbing alloys ofthe MmNi₅ type are preferably used. In the aforesaid alloys, Mm is amixture of rare earth elements such as La, Ce, Pr and Nd, and known asmisch metal.

[0021] Among MmNi₅ alloys, alloys of the general formula(La)_(x)R_(1−x)(Ni)_(a)(M)_(b) are preferably used in the presentinvention from the viewpoint of battery capacity and cycle lifecharacteristics. In this formula, R is a rare earth metal other than La,and preferably at least one element selected from Ce, Pr and Nd; M is atleast one element selected from Mn, Al, Co, Ti, Fe, Cu and Zr; and x, aand b are positive numbers. Preferably, x is in the range of 0.2 to 1, bsatisfies 0<b≦2.0, and (a+b) is in the range of 4.0 to 6.0.

[0022] In order to improve cycle life characteristics, M preferablycontains Cr or Al, and more preferably contains Mn in addition to Cr orAl.

[0023] In the hydrogen absorbing alloy of the present invention, thecomponent A, which is preferably a mixture of rare earth elements,contains 20 to 50% by weight of Ce. Moreover, in the present invention,a part of Ni may be replaced with Co or Al, and may further be replacedwith a metallic element such as Mn, Fe, Ti, Cu or Zr. When an alloyhaving such a Ce-rich composition is used to form negative electrodesfor use in nickel-metal hydride storage batteries, it becomes possibleto produce batteries having a long life.

[0024] Moreover, the component A also contains 20 to 50% by weight ofLa. This makes it possible to secure a high capacity.

[0025] The alloy used in the present invention is represented by AB₅ inwhich A is exothermic metal and B is endothermic metal. Moreparticularly, the rare earth metals contained therein may include 20 to50% by weight (preferably 40 to 50% by weight) of La, 20 to 50% byweight (preferably 30 to 40% by weight) of Ce, and the balancecomprising rare earth elements other than Ce and La and inclusive of Y.In particular, Pr or Nd is preferably selected and used as other rareearth elements. More preferably, the combined amount of La and Ce isdetermined so as to be in the range of 70 to 90% by weight, especiallyin that this brings about an improvement in both initial characteristicsand cycle life. The reason why the compositional range of A has beendefined as above is that, outside the above-defined range, a reductionin initial characteristics and capacity or a reduction in cycle life mayresult.

[0026] In the practice of the present invention, a hydrogen absorbingalloy is obtained, for example, by melting a mixture of metallicelements having the above-described composition in a well-knownhigh-frequency induction furnace or other furnace having an atmosphereof an inert gas. Then, using a ball mill, jet mill, pulverizer or thelike, this hydrogen absorbing alloy is reduced to a powder having apreferable average particle diameter of 5 to 50 μm, more preferably 15to 50 μm. Thereafter, this powder is soaked in an acid solution withstirring.

[0027] Alternatively, a hydrogen absorbing alloy powder producedaccording to a rapid quenching method such as a rapid roll quenching orgas atomization may also be used.

[0028] According to the present invention, in order to produce ahydrogen absorbing alloy powder which exhibits excellent initialcharacteristics, a high capacity and a long life when it is used inbatteries, the above-described hydrogen absorbing alloy is treated witha solution of an organic acid (in particular, a sulfur-containingorganic acid) and washed with water as required. Subsequently, thishydrogen absorbing alloy is treated with a phosphorus compound, whichmakes it possible to fabricate batteries having excellent initialcharacteristics.

[0029] In the practice of the present invention, a hydrogen absorbingalloy powder is treated with an acid solution, washed several times withwater to remove any excess acid from the alloy surface, and then treatedwith a phosphorus compound.

[0030] After treatment with the phosphorus compound, the hydrogenabsorbing alloy powder is dried without washing it with water, so thatthe phosphorus compound becomes adsorbed to the alloy surface and coversit. Thus, there is obtained a hydrogen absorbing alloy powder inaccordance with the present invention.

[0031] The acid solution used in the present invention preferablycomprises a solution containing an organic acid or its salt.

[0032] Preferably, the organic acid used in the present invention is anorganic acid having a sulfur-containing group. Such organic acidsinclude sulfonic acids (R¹—SO₃H), sulfinic acids (R²—SO₂H) and sulfenicacids (R³—SOH). In the formulae, R¹, R² and R³ each independentlyrepresent a monovalent aliphatic hydrocarbon radical (preferably having1 to 10 carbon atoms), an aromatic hydrocarbon radical, an amino groupor the like, and these radicals or groups may be substituted byhydrophilic groups such as carboxyl, amino, hydroxy and sulfonic acidgroups.

[0033] Specific examples of these organic acids include sulfamic acid,sulfanilic acid, sulfaminobenzoic acid, sulfosalicylic acid,sulfobenzoic acid and sulfoacetic acid.

[0034] These organic acids may be used irrespective of the position ofthe substituent group (i.e., they may include various isomers), and maybe used alone or in admixture.

[0035] Among these organic acids, sulfosalicylic acid, sulfamic acid andsulfoacetic acid are preferably used in the present invention.Especially preferred are 5-sulfosalicylic acid and2-hydroxyl-sulfobenzoic acid.

[0036] The organic acid salts which can be used in the present inventioninclude, for example, Na, K and Ca salts. Especially preferred are theNa, K and Ca salts of sulfosalicylic acid, sulfamic acid and sulfoaceticacid. In present invention, these organic acid salts may be used aloneor in admixture of two or more.

[0037] Although the acid solution used in the present inventionpreferably comprises a solution containing an organic acid or its salt,a solution containing both of them may also be used.

[0038] Examples of the solvent constituting the acid solution includewater, alcohols of 1 to 5 carbon atoms, ethers, and ketones (e.g.,acetone). From the viewpoint of workability, it is preferable that theacid solution be in the form of an aqueous solution.

[0039] When a solution of an organic acid is used as the acid solution,the concentration of the acid solution is preferably in the range of0.01 to 10% by weight.

[0040] The treatment with the acid solution used in the presentinvention is preferably carried out at a temperature ranging from roomtemperature (20° C.) to 100° C. and more preferably from 20 to 60° C. Ifdesired, the treatment may be carried out in a closed vessel under anelevated pressure up to 10 kgf/cm². For purposes of industrialproduction, treatment under heated or cooled conditions is undesirablefrom an economic point of view because a considerable equipment cost isrequired. In particular, treatment under cooled conditions is notpractical because an unduly long treating time is required.

[0041] Under any treating conditions, the treating time is in the rangeof 0.1 to 10 hours. The treating time may be suitably controlled byreducing it as the temperature becomes higher or extending it as thetemperature becomes lower.

[0042] The treatment with the acid solution used in the presentinvention is preferably carried out at a temperature ranging from roomtemperature to 100° C., and the treatment with a condensed phosphoricacid and/or phytic acid is preferably carried out at a temperatureranging from room temperature to 130° C.

[0043] In the treating bath containing an organic acid, the organic acidis preferably used in an amount of about 0.01 to 30 parts by weight(i.e., resulting in about 0.01 to 23% by weight), more preferably 0.5 to10 parts by weight, per 100 parts by weight of the hydrogen absorbingalloy being treated. Preferably, this organic acid is dissolved in asolvent so as to give a concentration of 0.01 to 10% by weight.

[0044] If the amount of the organic acid or its salt is greater than 30parts by weight, an improvement in initial characteristics can beachieved, but the attainable capacity may be reduced. If it is less than0.01 part by weight, the desired treating effect may not be obtained.

[0045] In the practice of the present invention, the hydrogen absorbingalloy having been treated with the acid solution is washed several timeswith water and then treated with a phosphorus compound. Specifically,the phosphorus compound preferably includes condensed phosphoric acid ofthe general formula H_(n+2)P_(n)O_(3n+1) and polymetaphosphoric acid ofthe general formula (HPO₃)_(m) which has a cyclic molecule formed by thecondensation of orthophosphoric acid, wherein n and m are each aninteger of 2 to 20, more preferably 2 to 10. The phosphorus acidcompound is preferably used in an amount of 0.01 to 30 parts by weightper 100 parts by weight of the alloy (i.e., resulting in about 0.01 to23% by weight). More preferably, it is used in an amount of 0.1 to 10parts by weight. If the amount of the phosphorus compound is undulysmall, the resulting alloy powder will have poor storability, while ifit is unduly great, the resulting alloy powder may conversely show areduction in the degree of activation.

[0046] The reason why a condensed phosphoric acid having 2 to 20phosphorus atoms is preferred is as follows. Phosphoric acid (H₃PO₄)improves initial activation to some extent, but tends to oxidize thealloy surfaces and thereby cause a reduction in attainable capacity. Ifthe number of phosphorus atoms is greater than 20, the hydrogenabsorbing and hydrogen-releasing reactions may be inhibited to cause adecrease in the amount of hydrogen absorbed.

[0047] In the present invention, the phosphorus compound is used in theform of a solution containing it. The solvents which can be used forthis purpose include water, alcohols (preferably having 1 to 5 carbonatoms), aromatic hydrocarbon solvents (e.g., toluene), and ketones(e.g., acetone). Moreover, solvent mixtures such as water-alcohol andacetone-toluene may also be used.

[0048] Preferably, the phosphorus compound is used in the form of asolution having a concentration of 0.01 to 10% by weight.

[0049] In the present invention, a mixture of such condensed phosphoricacids may be used. Moreover, phytic acid may also be used in combinationwith or in place of the condensed phosphoric acid.

[0050] The concentration of the phosphoric acid compound is preferablyin the range of about 0.01 to 10 parts by weight (i.e., resulting inabout 0.01 to 10% by weight), more preferably about 0.1 to 1 part byweight, per 100 parts by weight of the hydrogen absorbing alloy beingtreated. If the concentration is less than 0.01 part by weight, theresulting alloy powder will have poor storability, while if it isgreater than 10 parts by weight, the resulting alloy powder may show areduction in the degree of activation. The concentration of thecondensed phosphoric acid and/or phytic acid refers to the concentrationof the condensed phosphoric acid when the condensed phosphoric acid isused alone, the concentration of phytic acid when phytic acid is usedalone, or the combined concentration of the condensed phosphoric acidand phytic acid when they are used in combination.

[0051] The treatment with the phosphorus compound used in the presentinvention is preferably carried out at a temperature ranging from roomtemperature (20° C.) to 130° C. If desired, the treatment may be carriedout in a closed vessel under an elevated pressure up to 10 kgf/cm². Forpurposes of industrial production, treatment under heated or cooledconditions is undesirable from an economic point of view because aconsiderable equipment cost is required. In particular, treatment undercooled conditions is not practical because an unduly long treating timeis required.

[0052] Under any treating conditions, the treating time is in the rangeof 0.1 to 10 hours. The treating time may be suitably controlled byreducing it as the temperature becomes higher or extending it as thetemperature becomes lower.

[0053] No particular limitation is placed on the method for carrying outthe treatment with an acid solution (e.g., a solution of an organicacid) or the treatment with a solution containing a condensed phosphoricacid according to the present invention, and there may be employed anywell-known method that comprises, for example, soaking the hydrogenabsorbing alloy in the above-described solution. However, in order tocarry out the treatment satisfactorily, it is preferable to soak afinely divided powder of the hydrogen absorbing alloy in the treatingsolution. Especially when the treating solution is stirred, the initialcharacteristics of electrodes formed of the resulting powder can furtherbe improved.

[0054] Now, the method for forming negative electrodes for batteries byusing the hydrogen absorbing alloy powder produced according to thepresent invention.

[0055] A well-known binder such as polyvinyl alcohol, methylcellulose,carboxymethylcellulose, PTFE (polytetrafluoroethylene) or high polymerlatex is added to the hydrogen absorbing alloy powder in an amount of0.1 to 20 parts by weight per 100 parts by weight of the alloy, andmixed therewith to form a paste. If necessary, 0.5 to 20 parts by weightof an electrically conducting filler such as carbon-graphite powder, Nipowder or Cu powder may also be added thereto. Thereafter,three-dimensional electrical conductors (e.g., porous bodies of foamednickel or bodies made of Ni fibers) or two-dimensional electricalconductors (e.g., punching metals) are uniformly filled with the paste,dried in vacuo, and then pressed to form negative electrodes forbatteries in accordance with the present invention. Such a negativeelectrode, together with a well-known positive electrode formed ofnickel, a separator (e.g., polypropylene) and an electrolytic solution(e.g., a KOH solution), may be incorporated into a container tofabricate a nickel-metal hydride storage battery.

[0056] When the hydrogen absorbing alloy having undergone the acidtreatment is used to form negative electrodes for batteries, animprovement in initial characteristics can be achieved. The reason forthis is presumed to be that the oxide film present on the surface of thehydrogen absorbing alloy is removed by the acid to establish intimatecontact between the alloy and the electrolytic solution, a Ni-rich layeris formed on the surface of alloy particles by the acid treatment, andthe alloy surface is coated with the phosphorus compound adsorbedthereto.

[0057] According to the present invention, a hydrogen absorbing alloypowder which not only has a high activity but also exhibits excellentstorability and handleability can be produced very easily. Moreover,nickel-metal hydride storage batteries having excellent initialcharacteristics can be fabricated by using the hydrogen absorbing alloypowder produced according to the present invention.

[0058] In addition to the above-described formation of electrodes, thehydrogen absorbing alloy powder produced according to the presentinvention may also be used as a hydrogen storage means for heat pumpsand the like.

[0059] The present invention is further illustrated by the followingexamples. However, these examples are not to be construed to limit thescope of the invention.

EXAMPLES 1-8

[0060] Various alloy components were mixed in such proportions as togive an alloy composition represented byR′_(1.0)Ni_(3.8)Co_(0.7)Mn_(0.2)Al_(0.3) in which R′ contains 20-50 wt %La and 20-50 wt % Ce. These mixtures were melted in a high-frequencymelting furnace having an atmosphere of argon to yield a total of eighthydrogen absorbing alloys.

[0061] Subsequently, each of these hydrogen absorbing alloys washeat-treated at 1,050° C. for 8 hours in an atmosphere of argon, andthen pulverized to yield a hydrogen absorbing alloy powder having anaverage particle diameter of 40 μm.

[0062] Moreover, this hydrogen absorbing alloy powder was subjected to asurface treatment. This surface treatment was carried out by treatingthe powder with sulfosalicylic acid, washing it with water, and thentreating it with a condensed phosphoric acid. The conditions for thesulfosalicylic acid treatment were such that a 5 wt % aqueous solutionof sulfosalicylic acid was added to the powder in an amount of 36milliliter per 30 g of the hydrogen absorbing alloy, and stirred at 60°C. for 15 minutes. After the sulfosalicylic acid treatment, the powderwas washed twice with deionized water. Then, polyphosphoric acid(H₆P₄O₁₃) was added to the powder in an amount of 0.5 wt % based on thehydrogen absorbing alloy, and stirred at room temperature for 30minutes. The powder was separated by decantation and dried in a vacuumdryer.

COMPARATIVE EXAMPLES 1-4

[0063] Various alloy components were mixed in such proportions as togive an alloy composition represented byR′_(1.0)Ni_(3.8)Co_(0.7)Mn_(0.2)Al_(0.3) in which R′ comprises 100 wt %La or 50 wt % La and 50 wt % Ce, Nd or Pr. These mixtures were worked upin the same manner as in Examples 1-8. Thus, there were obtained a totalof four hydrogen absorbing alloy powders having an average particlediameter of 40 μm. However, these hydrogen absorbing alloy powders werenot subjected to any surface treatment with sulfosalicylic acid orpolyphosphoric acid.

COMPARATIVE EXAMPLES 5-8

[0064] Various alloy components were mixed in such proportions as togive an alloy composition represented byR′_(1.0)Ni_(3.8)Co_(0.7)Mn_(0.2)Al_(0.3) in which R′ comprises 20-80 wt% La and 20-80 wt % Ce. These mixtures were worked up in the same manneras in Examples 1-8. Thus, there were obtained a total of four hydrogenabsorbing alloy powders having an average particle diameter of 40 μm.However, these hydrogen absorbing alloy powders were not subjected toany surface treatment with sulfosalicylic acid or polyphosphoric acid.

EXAMPE 9 and COMPARATIVE EXAMPLES 9-10

[0065] Various alloy components were mixed in such proportions as togive an alloy composition represented byR′_(1.0)Ni_(3.8)Co_(0.7)Mn_(0.2)Al_(0.3) in which R′ comprises 61 wt %La and 7 wt % Ce etc. (Example 9) or 50 wt % La and 50 wt % Ce(Comparative Examples 9 and 10). These mixtures were worked up in thesame manner as in Examples 1-8. Thus, there were obtained a total ofthree hydrogen absorbing alloy powders having an average particlediameter of 40 μm. The hydrogen absorbing alloy powder of Example 9 wassubjected to a surface treatment with sulfosalicylic acid andpolyphosphoric acid. However, the hydrogen absorbing alloy powder ofComparative Example 9 was treated with polyphosphoric acid alone, andthat of Comparative Example 10 was treated with sulfosalicylic acidalone.

[0066] Construction of Open Type Batteries

[0067] A 2-g sample was taken from each of the treated alloy powders.0.5 g of a 3 wt % aqueous solution of polyvinyl alcohol (with a degreeof saponification of 98 mole % and an average degree of polymerizationof 2,000) was added thereto and mixed therewith to form a paste. Thus,there were obtained a total of 19 pastes (Examples 1-9 and ComparativeExamples 1-10).

[0068] A porous plate of foamed nickel measuring 30 mm×40 mm×1.2 mm andhaving a porosity of 94-96% was uniformly filled with each of the above19 pastes, dried in vacuo, and then pressed. Thus, a total of 19negative electrodes were formed.

[0069] On the other hand, positive electrodes of nickel oxide wereformed of sintered nickel prepared according to a well-known method.

[0070] Using separators formed of a polyolefin nonwoven fabric, theaforesaid positive electrodes were combined with the 19 negativeelectrodes (Examples 1-9 and Comparative Examples 1-10). Moreover, a 6Naqueous solution of potassium hydroxide was used as an electrolyticsolution. Thus, a total of 19 open type nickel-metal hydride storagebatteries were constructed.

[0071] A charged positive electrode was employed as a referenceelectrode and used in such a way as to undergo no influence of thepositive electrode.

[0072] Evaluation of Capacity and Cycle Performance

[0073] Each of the batteries so constructed was charged at a constanttemperature of 20° C. and a charging rate of 0.3 C (180 mA) for 5 hours,and discharged at 0.2 C (120 mA) until the battery voltage reached 0.8V. This procedure was regarded as one cycle and repeated for testingpurposes. The initial activity performance was evaluated by measuringthe capacities in cycle 1 and cycle 10. Moreover, the cycle test wasfurther repeated to measure the capacity after 300 cycles. Thus, theretention of capacity after repeated charging-discharging cycles, whichis defined as the ratio of the capacity after 300 cycles to the maximumcapacity, was calculated according to the following equation.

Retention of capacity (%) ={(Capacity after 300 cycles)/(Maximumcapacity)}×100

[0074] The results thus obtained are shown in Table 1. TABLE 1 DischargeDischarge capacity in capacity in Retention of Composition of R′ Alloycomposition Phosphorus cycle cycle 10 capacity after (weight ratio)(atomic ratio) Acid treatment compound 1 (mAh/g) (mAh/g) 300 cycles (%)Example 1 La0.5Ce0.5 R′1.0Ni3.8Co0.7Mn0.2Al0.3 sulfosalicyclic acidH₆P₄O₁₃ 270 304 94 Example 2 La0.4Ce0.5Pr0.1 R′1.0Ni3.8Co0.7Mn0.2Al0.3sulfosalicyclic acid H₆P₄O₁₃ 268 304 95 Example 3 La0.3Ce0.5Pr0.2R′1.0Ni3.8Co0.7Mn0.2Al0.3 sulfosalicyclic acid H₆P₄O₁₃ 262 303 95Example 4 La0.2Ce0.5Pr0.3 R′1.0Ni3.8Co0.7Mn0.2Al0.3 sulfosalicyclic acidH₆P₄O₁₃ 245 300 96 Example 5 La0.5Ce0.4Pr0.1 R′1.0Ni3.8Co0.7Mn0.2Al0.3sulfosalicyclic acid H₆P₄O₁₃ 266 304 95 Example 6 La0.5Ce0.3Pr0.2R′1.0Ni3.8Co0.7Mn0.2Al0.3 sulfosalicyclic acid H₆P₄O₁₃ 262 304 95Example 7 La0.5Ce0.2Pr0.3 R′1.0Ni3.8Co0.7Mn0.2Al0.3 sulfosalicyclic acidH₆P₄O₁₃ 261 303 94 Example 8 La0.4Ce0.4Nd0.2 R′1.0Ni3.8Co0.7Mn0.2Al0.3sulfosalicyclic acid H₆P₄O₁₃ 255 302 96 Example 9La0.61Ce0.07Pr0.23Nd0.09 R′1.0Ni3.8Co0.7Mn0.2Al0.3 sulfosalicyclic acidH₆P₄O₁₃ 274 303 65 Com. Ex. 1 La1.0 R′1.0Ni3.8Co0.7Mn0.2Al0.3 — — 231335 32 Com. Ex. 2 La0.5Pr0.5 R′1.0Ni3.8Co0.7Mn0.2Al0.3 — — 158 298 91Com. Ex. 3 La0.5Nd0.5 R′1.0Ni3.8Co0.7Mn0.2Al0.3 — — 152 296 88 Com. Ex.4 La0.5Ce0.5 R′1.0Ni3.8Co0.7Mn0.2Al0.3 — — 172 304 92 Com. Ex. 5La0.8Ce0.2 R′1.0Ni3.8Co0.7Mn0.2Al0.3 — — 184 304 88 Com. Ex. 6La0.6Ce0.4 R′1.0Ni3.8Co0.7Mn0.2Al0.3 — — 178 303 90 Com. Ex. 7La0.4Ce0.6 R′1.0Ni3.8Co0.7Mn0.2Al0.3 — — 164 298 93 Com. Ex. 8La0.2Ce0.8 R′1.0Ni3.8Co0.7Mn0.2Al0.3 — — 152 290 93 Com. Ex. 9La0.5Ce0.5 R′1.0Ni3.8Co0.7Mn0.2Al0.3 — H₆P₄O₁₃ 176 304 93 Com. Ex. 10La0.5Ce0.5 R′1.0Ni3.8Co0.7Mn0.2Al0.3 sulfosalicyclic acid — 196 300 91

[0075] It can be seen from the results of Comparative Example 1 that,when R′ comprises 100% by weight of La, the discharge capacities incycle 1 and cycle 10 are high, but the cycle life is extremely poor. Ithas been confirmed by the results of Comparative Examples 2 and 3 thatan improvement in initial capacity and cycle life can be achieved byreplacing a part of La with Ce.

[0076] Moreover, with respect to each of the above-described samples,the alloy of the negative electrode having undergone 300 cycles wasobserved by SEM and XPS. As a result, it has been found that the alloy(a La—Ce mixture) of the negative electrode of Comparative Example 4 hasthe largest particle size, while the alloy (100% by weight of La) of thenegative electrode of Comparative Example 1 has the smallest particlesize. This indicates that the degree of particle size reduction variesaccording to the negative electrode. Moreover, XPS has revealed that ahigh concentration of Co is present on the surface of the alloy ofComparative Example 4. Since the alloys in which a part of La isreplaced with Ce show an improvement in cycle life, it is believed thatCe has distinct superiority to other light rare earth elements.

[0077] It can also be seen that, when 20% by weight or more of Lapresent in a LaNi₅ alloy is replaced with Ce, the resulting alloy showsan improvement in cycle life. However, when 50% by weight or more of Lais replaced with Ce, the resulting alloy shows an improvement in life,but it also suffers from a retardation of initial activation and areduction in initial capacity and maximum capacity. The reason for thisis presumed to be that the particle size reduction of the alloy isgreatly inhibited owing to the degree of replacement with Ce, resultingin poor activation and reduced maximum capacity.

[0078] Accordingly, the amount of Ce substituting for La should belimited within the range of 20 to 50% by weight. It is assumed that analloy having a high capacity and a long life can preferably be obtainedby using 30 to 40% by weight of Ce.

[0079] It is especially preferable that both La and Ce are present in anamount of 20 to 50% by weight, respectively. If the amount of La isgreater than 50% by weight, the resulting alloy will be analogous toLaNi₅ alloy. When a negative electrode is formed of this alloy, theresulting alkaline absorbing battery has an extremely shortcharging-discharging cycle life and is hence disadvantageous from aneconomic point of view. If the amount of La is less than 20% by weight,the amount of hydrogen absorbed will be insufficient. When an alloyhaving the above-described optimum La—Ce composition is subjected tosurface treatment steps, a further improvement in initial capacity andlife can be achieved.

[0080] The above-described results demonstrate the effectiveness of thepresent invention.

1. A process for the production of a hydrogen absorbing alloy powderwhich comprises steps of treating a hydrogen absorbing alloy powder withan acid solution, and subsequently treating the hydrogen absorbing alloypowder with a solution containing a condensed phosphoric acid having 2to 20 phosphorus atoms per molecule and/or phytic acid.
 2. A process forthe production of a hydrogen absorbing alloy powder as claimed in claim1 wherein the acid solution is a solution containing at least onecompound selected from the group consisting of sulfonic acids, sulfinicacids, sulfenic acids and salts thereof.
 3. A process for the productionof a hydrogen absorbing alloy powder as claimed in claim 1 wherein thetotal amount of condensed phosphoric acid and/or phytic acid used is inthe range of 0.01 to 10 parts by weight per 100 parts by weight of thehydrogen absorbing alloy.
 4. A process for the production of a hydrogenabsorbing alloy powder as claimed in claim 2 wherein the total amount ofcondensed phosphoric acid and/or phytic acid used is in the range of0.01 to 10 parts by weight per 100 parts by weight of the hydrogenabsorbing alloy.
 5. An electrode formed of the hydrogen absorbing alloypowder produced by a process as claimed in claim 1 .
 6. An electrodeformed of the hydrogen absorbing alloy powder produced by a process asclaimed in claim 2 .
 7. An electrode formed of the hydrogen absorbingalloy powder produced by a process as claimed in claim 3 .
 8. Anelectrode formed of the hydrogen absorbing alloy powder produced by aprocess as claimed in claim 4 .
 9. A hydrogen absorbing alloy powder ofthe AB₅ type in which A is exothermic metal and B is endothermic metal,the hydrogen absorbing alloy powder being obtained by providing ahydrogen absorbing alloy in which A comprises a mixture composed of 20to 50% by weight of Ce, 20 to 50% by weight of La, and the balancecomprising rare earth elements other than Ce and La and inclusive of Y(yttrium), treating the hydrogen absorbing alloy with an acid solution,and subsequently treating the hydrogen absorbing alloy with a solutioncontaining a phosphorus compound.
 10. A hydrogen absorbing alloy powderas claimed in claim 9 wherein the acid solution is a solution of anorganic acid and the phosphorus compound is a condensed phosphoric acidhaving 2 to 20 phosphorus atoms.
 11. An negative electrode for use innickel-metal hydride batteries which is formed of a hydrogen absorbingalloy powder as claimed in claim 9 .
 12. An negative electrode for usein nickel-metal hydride batteries which is formed of a hydrogenabsorbing alloy powder as claimed in claim 10 .